I would agree that 100k does sound a bit high. I would use 10k as a starting point (befor considering any other reasons to go up or down).

It should only really be a problem if there is a lot of capacitance on the line. Of course, adding a CRO probe will be adding some more capacitance to that but it should be relatively insignificant.

Perhaps something else wasn't playing ball originally? The main reason I can think of is perhaps it something couldn't sink enough current.

Or perhaps something has introduced some capacitance to this particular empeg?

I2C has pretty solid requirements so normally this problem shouldn't ever happen.